Among various optical access networks, there are those known more particularly passive optical networks (PON) in a star configuration as shown in FIG. 1, that are based on a passive point-to-multipoint optical fiber architecture as described in particular by D. W. Faulkner and A. L. Harmer, “Broadband access networks”, Networks and Optical Communications (NOC'97), Book, IOS, 1997, and by M. Nakamura, H. Ueda, S. Makino, T. Yokotani, and K. Oshima in “Proposal of networking by PON technologies for full land Ethernet services in FTTx”, Journal of Lightwave Technology, Volume 22, Issue 11, pp. 2631-2640, November 2004.
The optical access network AN provides the link between user networks UN1, . . . , UNN and the core network CN of the communications network. The PON type optical access network begins at the core network end with optical link terminal (OLT) active equipment that is generally located in a central office CO and that sends and receives data-carrying light signals. At the user network end, the network terminates in optical network terminals (ONTs) that are sometimes referred to as optical network units (ONUs), it being understood that there are as many ONTs as there are user networks UN1, . . . , UNN.
The signal referred to as a “downlink” signal is a signal that propagates from the central office towards a client terminal forming part of the user network, and the signal referred to as the “uplink” signal is a signal that propagates in the opposite direction.
The PON architecture relies on using a passive optical coupler/combiner known as a splitter (SP), i.e. there is no need for it to be electrically powered, thus making it possible to use a 1-to-N topology using an uplink wavelength and a downlink wavelength. Each port of the OLT is connected to an optical coupler SP by a single optical fiber OF. The operation of the coupler SP is based solely on propagation within optical fibers. In the downlink direction, the coupler SP splits the optical signal coming from the OLT into secondary optical fibers OF1 to OFN so that it goes towards the various ONTs. Each ONT then filters the signal destined to the single associated user UN1, . . . , or UNN. In the uplink direction, optical signals coming from the users are combined by each user complying with an access protocol, such as time division multiple access (TDMA), etc. Usually, all of the ONTs transmit at the same wavelength for reasons of cost.
Optical access networks are the subject of recommendations or standards that have already been published or that are being prepared within the Institute of Electrical and Electronic Engineers (IEEE) and the International Telecommunication Union Telecommunication Standardization Sector (ITU-T), and more particularly within the Full Services Access Network (FSAN) group. Among the various standards, the Ethernet and Gigabit passive optical network (EPON and GPON) standards are presently emerging: both of them serve to reach rates greater than 1 Gbit/s. Thus a G-PON network can deliver rates of 2.5 Gbit/s in the downlink direction and 1.25 Gbit/s in the uplink direction for 32 or 64 clients. An access network seeking to achieve 10 Gbit/s transmission is being finalized within IEEE 802.3, ITU-T SG15 Q2, and the FSAN group for an Ethernet PON version.
In parallel with those bodies, exchanges are taking place between economic players that are leading to the definition of a wavelength division multiplex (WDM) PON. Such a network is characterized by each client terminal corresponding to a signal transmission wavelength allocated by the central office. With reference to FIG. 2, the OLT has a multiplexer/demultiplexer that performs wavelength multiplexing/demultiplexing for each client terminal, and the coupler is replaced by a client access node RN that includes a multiplexer/demultiplexer performing wavelength multiplexing/demultiplexing for each client. Given that the ONTs need to be manufactured in large numbers, their cost influences the technological choices that are made for deploying optical access networks. In particular, the technology that makes it possible to obtain identical ONTs while still being compatible with an uplink wavelength that is configurable and specific to each user may turn out to be a good candidate for such networks. It is thus generally considered that ONTs should be “colorless”.
A recent so-called “self-seeded” technology, an implementation of which is described in [1], enables the allocation of wavelengths to be self-organizing in passive manner, more particularly for uplink signals, and thus for the emitter modules that are to be found in the ONTs of a WDM PON access network. That technique is shown in FIG. 3, and it is based on adding a partially reflective optical component in the laser cavity of each ONT emitter module, which component is typically a mirror MR. The partially reflective optical component enables the light source OptGain to self-seed its emission wavelength on the principle of light going and returning between the mirror MR and the source OptGain, which is a medium having optical gain. The partially reflective mirror MR necessarily operates partially in transmission. Typically, the light source is a reflective semiconductor optical amplifier (RSOA) or a semiconductor optical amplifier (SOA) associated with a reflective modulator. The wavelength is selected by an optical filter, generally an arrayed wavelength grating (AWG) that co-operates with the RSOA source.
A system using that self-seeded technique is shown in FIG. 4. The central office CO has 32 pieces of terminal equipment OLT, each having a self-seeded RSOA source, i.e. a source co-operating with an optical filter and a semireflective optical component that co-operates with the source to define the laser cavity, the filter being in the cavity. Each OLT also has a receiver for detecting the uplink signal coming from an ONT. The optical filter is an AWG having 32 channels associated with a single semireflective mirror. The architecture of the RN client access node is symmetrical with that of the central office CO: 32 optical units ONT, each having a self-seeded RSOA light source and a receiver for detecting the downlink signal coming from an OLT. The self-seeded RSOA sources co-operate with an optical filter, a 32-channel AWG, and a semireflective mirror MR. The RSOA light sources emit in the L and C bands respectively for the uplink and downlink signals.
The principle of wavelength self-seeding on which the technique is based suffers from the drawback of introducing optical losses between the mirror and the light source. In order to obtain the laser effect, it is necessary for the gain that is delivered by the optical medium to be greater than the losses due to the various optical elements.